One goal of a wind farm controller is to control centrally an active and reactive power injected by a whole wind farm (also referred to as “wind park”) into a grid. This provides the possibility to participate actively in control tasks on the grid for the wind farms in the same way as conventional power plants do. Thereby, a wind farm control level behaves as a single centralized unit (“central wind farm control level”) which has as input, e.g., system operator orders, measurements from a power common coupling (“PCC”) and available powers from the wind turbines of the wind farm and as outputs elaborated reference information or signals for each individual wind turbine, i.e. each individual wind turbine control (“local wind turbine control level”).
Active power control functions of the wind farm controller may comprise, e.g., an automatic frequency control wherein the frequency measured in the wind farm point of common coupling (PCC) is controlled. The wind farm must thus be able to produce more or less active power in order to compensate for a deviant behavior in the frequency.
Reactive power control functions of the wind farm controller may comprise, e.g., an automatic voltage control wherein the voltage in the wind farm point of common coupling (PCC) is controlled. This implies that the wind farm can be ordered to produce or absorb an amount of reactive power to the grid in order to compensate for the deviations in the voltage in the grid.
One reason for placing automatic frequency control in the wind farm control level is to avoid that the wind farm controller can counteract the frequency control implemented in the individual wind turbine. Automatic voltage control is placed in the wind farm control level in order to avoid a risk of instability and a high flow of reactive power between the wind turbines. Usually, the implementation of both frequency and voltage control is going to be done as a combined droop and dead band control.
The central wind farm control level may comprise two separated control loops, one for the active power control and the other for the reactive power control.
One possible implementation of the active and reactive control loop may be as follows:
First, an active and reactive power reference signal, respectively, are derived in a control function block, based on one or several control functions required by the system operator. These reference signals may be, if necessary, adjusted further with some corrections from subordinated control loops (e.g. focusing on frequency and voltage) respectively, in order to assure that the frequency and voltage limits in the PCC are not violated. Each loop consists of a PI controller ensuring a correct power production from the wind farm. The controller computes a power error and sets up the power reference for the whole wind farm. These power references are further converted into power reference signals for each individual wind turbine of the wind farm.
Controlling reactive power in a wind farm may be based on, e.g., a common voltage reference provided to the wind turbines.
When controlling reactive power in a wind farm using such a common voltage reference, the individual wind turbines may produce/consume different amounts of reactive power, depending on their location in the wind farm, measurement tolerances and other factors like, e.g., converter control strategy. This unbalance may lead to a loss of energy in the wind farm and unnecessary wear on components shorting their operational lifetime.
One possible solution for controlling reactive power within a wind farm may be based on distributing a reactive power reference to the wind turbines instead of a voltage reference. This solution may reduce the performance of responses to power grid events and the response times to changes in grid voltage in wind farms under voltage control.